1. Field of the Invention
The present invention relates to a method of milking an animal by a milking machine having a teatcup with a teatcup liner and a pulsation chamber, comprising applying the teatcup to the teat of the animal, and applying a milking vacuum to the interior of the teatcup liner and a pulsating vacuum to the pulsation chamber so that the teatcup liner cyclically opens and closes. Moreover the invention relates to a milking machine comprising a teatcup having a teatcup liner and a pulsation chamber, a vacuum source for generating a milking vacuum in the interior of the teatcup liner and a pulsator provided to alternately connect the pulsation chamber to the atmosphere and to said vacuum source for generating a pulsating vacuum in the pulsation chamber so that the teatcup liner cyclically opens and closes.
2. Description of the Prior Art
Traditionally a milking machine comprises a cluster having a claw and four teatcups, each teatcup having a shell and a teatcup liner provided in the shell to form a pulsation chamber between the teatcup liner and the shell. During milking the interior of the teatcup liner is subjected to a milking vacuum, i.e. a low pressure of normally about 50 kPa under atmospheric pressure. There are also milking machines working under high pressure conditions, whereby the low pressure might be above atmospheric pressure. The pulsation chamber is subjected to a cyclically pulsating vacuum normally varying between atmospheric pressure, when the teatcup liner is collapsed or closed, and a maximum vacuum level when the teatcup liner is fully open. The maximum pulsating vacuum level is normally a low pressure level of 50 kPa under atmospheric pressure, i.e. equal to the milking vacuum level. This means that the pressure difference across the wall of the teatcup liner is essentially equal to zero when the teatcup liner is fully open.
The pulsating vacuum demonstrates a pulsation cycle which may be divided into four phases, i.e. (a) an opening phase during which the pulsating vacuum increases from atmospheric pressure to normally about the milking vacuum level and the teatcup liner moves from a closed position to an open position, (b) an open phase during which the pulsating vacuum has reached its maximum level and is essentially equal to the milking vacuum level and the teatcup liner is in an open position, (c) a closing phase during which the pulsating vacuum decreases from about the milking vacuum level to the atmospheric pressure and the teatcup liner moves from the open position state to the closed position, and finally (d) a closed phase during which the pulsating vacuum is equal to the atmospheric pressure and the teatcup liner is in a closed position. The opening and closing of the teatcup liner during phase (a) and (c), respectively comprises a very fast and abrupt movement of the teatcup liner. From a closed state, i.e. opposite wall portions of the teatcup liner touch each other, as the pulsating vacuum increases the teatcup liner remains essentially closed until it at a certain pulsating vacuum level, the so called TPD (touch pressure difference), starts to open abruptly to the so called CCPD (critical collapse pressure difference) at which level the teatcup liner is fully open, i.e. said opposite wall portions of the teatcup liner are spaced apart from each other. From the point of time when the pulsating vacuum exceeds the CCPD-level the teatcup liner thus is essentially open and a further increase of the pulsating vacuum only results in an insignificant further opening of the teatcup liner.
Each milking may be divided into four periods, i.e. (I) the initial stimulation or massage period, (II) the main flow period, (III) the flow decreasing period, and (IV) the flow terminating period. During the initial, flow decreasing and flow terminating period the milk flow is reduced in comparison with the main flow period.
Among those skilled in the art it has up to now been considered necessary that the teatcup liners must be fully open once every pulsation cycle at least during the main flow period in order not to restrict the milk flow, and hence that the maximum pulsating vacuum level should be equal to or even higher than the milking vacuum level.
However the opening movement of the teatcup liner leads to a fast increase of the volume in the interior of the teatcup liner. During the initial, the flow decreasing and the flow terminating periods of each milking, i.e. when the milk flow is small, this volume increase in the interior of the teatcup liner will result in a momentary increase of the vacuum in the interior of the teatcup liner. Such a momentary vacuum increase may subject the exposed teat surface to a strong vacuum, resulting in stretching of the teat surface, and may lead to a back flow of the milk or of milk droplets which are jetted with a very high velocity towards and against the teat tip. This is a rather ungentle treatment which may hurt the animal, lead to injuries on the teats, and furthermore the backflow of milk increases the risk of transmitting diseases, e.g. mastitis. It may happen that bacteria associated with one teat passes directly into the interior of another teat, increasing the risk of infection.
Such a vacuum increase during in particular the terminating period may also result in that the teatcup is crawling upwards on the teat. Such upward movements of the teatcup at the end of the milking as the teat becomes slack leads to a throttling of the milk conducting interior of the teat and consequently the milk flow may be stopped although a considerable amount of rest milk remains in the udder.
Many proposals have been made in the past in order to remedy some or all of the problems discussed above.
WO-A-9 212 625 discloses a method of operating a milking machine such that the pulsating vacuum increase and decrease are changed at a slower rate during the massage period, the flow decreasing period and the flow terminating period. During the main flow period the pulsating vacuum changes at a normal rate. By means of this way of operating the milking machine, the teatcup liner moves more slowly when the milk flow is reduced, which results in less crawling and a more gentle treatment of the teats.
U.S. Pat. No. 4,211,184 discloses a method and an apparatus for milking with the aim to minimise vacuum damages to the teats of the animal. This document proposes to monitor the vacuum level in a ring chamber of the mouth piece of the teatcup and to control the pulsating vacuum in response thereto. The disclosed method and apparatus is described to operate such that the full milking vacuum level will be experienced in the ring chamber in the initial period to control the maximum pulsating vacuum to be at a low level, thus closing the teatcup liner, preventing the teatcup liner from crawling and preventing strong milking vacuum from damaging the teat. At full milk flow the teat will be pressurized by the presence of milk in the teat resulting in a reduced vacuum level in the ring chamber and thus an increase in the maximum pulsating vacuum level. At the flow terminating period the vacuum level in the ring chamber will again increase, which once again reduces the maximum pulsating vacuum level.
EP-A-584 890 and SE-B-382 547 both relate to the control of the pulsating vacuum in response to the milk flow which is detected by a milk flow sensor. EP-A-584 890 proposes to adjust the pulsator ratio in response to the milk flow such that the closed phase (d) is extended when the milk flow is reduced. SE-B-382 547 proposes to reduce the maximum pulsating vacuum level when the milk flow is less than a certain value.
WO-A-9 314 625 discloses a milking apparatus including, for each claw, a pulsator and a control unit responsive to the opening and closing of the teatcup liners. The pulsation switching characteristics of the pulsator are controlled by the control unit in sympathy with the opening and the closing of the teatcup liners in order to improve the pulsation effectiveness, i.e to ensure the fully collapsing and opening of the teatcup liners in a single pulsation cycle. Thus this piece of prior art provides a method in which the pulsator is not switched from the atmospheric pressure to a low pressure or vice versa until the liner is fully closed or opened, respectively. When the liners are fully closed or opened, the air flow in the pulsating vacuum pipe ceases. This cessation of air flow is detected by different means proposed in WO-A-9314625. E.g. such means comprises a piston and cylinder device provided in the pulsating vacuum pipe, whereby the piston moves with the air flow and comes to rest at two opposite positions at which positions the piston is detected to give a signal to the pulsator to change between atmospheric pressure and low pressure. Other proposed means are a device comprising a rubber diaphragm moving in response to the air flow, a rotating turbine device, and a hot wire galvanometer.